“Robert S. Mueller III, the F.B.I. director, said law enforcement officials had decided before Sept. 11 that they lacked legal authority to obtain a court-approved intelligence warrant, known as a F.I.S.A. warrant, to search [potential terrorist] Mr. [Zacarias] Moussaoui’s laptop computer [following his arrest on August 16, 2001]. It was later found to contain data concerning flight training. Asked if the bureau could have headed off the attack by investigating Mr. Moussaoui more extensively, Mr. Mueller said, ‘All I can tell you is that the agents on the scene attempted to follow up aggressively. The attorneys back at the F.B.I. determined that there was insufficient probable cause for a F.I.S.A., which appears to be an accurate decision.’ ”
– David Johnston and Philip Shenon, “Man Held Since August is Charged with a Role in Sept. 11 Terror Plot,” The New York Times, Dec. 12, 2001